As methods for compressing and expanding image data constituted by a still image, there are widely known JPEG (Joint Photographic Expert Group) standards standardized according to CCITT (International Telegraph and Telephone Consultative Committee) and ISO (International Standards Organization). In the JPEG standards, an image data compression method performed by dividing a frame image into a plurality of blocks in such a way that 8×8 pixels constitute one block, and transforming a spatial coordinate into a frequency coordinate, and an image data expansion method thereof are defined.
A data compressor according to the JPEG standards (hereinafter call “JPEG compressor”) divides input image data into many blocks, and performs a DCT (Discrete Cosine Transform) processing and a quantization processing on each block. In this quantization processing, a value obtained by multiplying data specified for each DCT coefficient by a quantization table by a quantization factor Q is used as a quantization step width. The DCT coefficient obtained by the DCT processing is quantized by the quantization step width, thereby irreversibly reducing a data amount. Thereafter, entropy coding using a run-length processing, a differential processing, a Huffman coding processing or the like is performed, thereby generating compressed data. This coding is a processing for irreversibly reducing the data amount.
On the other hand, a data expander according to the JPEG standards (hereinafter call “JPEG expander”) performs opposite processings to those performed by the JPEG compressor to restore compressed image data to original image data. Namely, input compressed image data is decoded and dequantized using the same quantization table and the same quantization factor Q as those used in the data compression. Thereafter, an inverse DCT processing unit performs an inverse DCT transform to combine the divided blocks, thereby restoring the compressed image data to the original image data.
To improve a data compression rate of the above-mentioned JPEG compressor, it is necessary to change the quantization table or the quantization factor Q so as to make the quantization step width larger. However, if a large data amount is reduced in the quantization processing, which is an irreversible processing, a quality of the restored image data is greatly degraded. In addition, this quality degradation occurs throughout the image. Due to this, even if an important region and an unimportant region are present in the image, an image quality is disadvantageously, uniformly degraded in the both regions.
To solve this problem, the applicant of the present application filed prior patent applications (identified as Japanese Patent Application No. 2003-43367, 2004-040643, and 2004-041212). These prior applications disclose a technique for dividing image data into a plurality of regions, and for downsampling the image data for a part of the regions as a preprocessing to a JPEG compression processing so as to degrade a quality of the image only for the unimportant region and to thereby reduce an amount of data that has been subjected to the JPEG compression processing. This downsampling is a processing for downsampling the image data in the regions to reduce the image data, and for inserting fill bits into the remaining portion of each of the regions.
Furthermore, to solve the above-mentioned problem, there is proposed, as a conventional technique, a compression processing method for making a quality of an image after restoration different among regions of the image (see, for example, Patent Document 1). The Patent Document 1 discloses an image compression apparatus that includes a mask circuit that masks a DCT coefficient before a quantization processing. This data compression apparatus makes a mask employed in the mask circuit different among the regions, thereby coding an image in the important region at a high image quality and coding the image in the unimportant region at a low image quality.
Patent Document 1: Japanese Unexamined Patent Publication No. 1994-054310